The problem which must be solved by means of the sterilization and/or pasteurization of these products essentially consists in attenuating the pathogenic and enzyme activity of the micro-organisms while respecting the active principles which determine the original organic and organoleptic properties.
Known preservation methods used on an industrial scale are of the physical, chemical-physical, chemical and biological types. In particular, physical methods comprise refrigeration, heat transmission or drying.
Among the above mentioned methods, processes based on the application of heat are by far the most valid and widespread in solving the above mentioned problems. Heat application conditions depend not only on the type of product to be treated but also on the types of micro-organism contained therein and finally on the simultaneous use or non-use of other preservation processes. The degree of heat resistance of the micro-organisms must be related both to external and environmental factors, such as the initial microbial concentration of the medium, the characteristics of said medium and the time and temperature parameters, and to intrinsic factors, such as the heat resistance of the germs.
Sterilization destroys all of the micro-organisms present in the products by heating the product up to temperatures between 65.degree. and 121.degree. C. for a time between 5 and 12 minutes.
Pasteurization comprises a moderate heat treatment in order to destroy most, but not all, of the bacterial flora by using temperatures below 100.degree. C., generally between 60.degree. and 75.degree. C.
In both cases, the temperature and duration of the heat treatment depend on the heat application method and on the type of product. Furthermore, at the end of the treatment the product must be subjected to the fastest possible cooling down to temperatures below 35.degree. C. before being introduced in sterilized containers.
In the sterilization process as well as in the pasteurization process, heat can be applied with an indirect exchange, in which the product and the heating medium are separated by the wall of an exchanger, or with a direct one, in which the product and the heating medium are in direct contact.
Current thermal preservation methods are the most important from the industrial point of view, but they have some problems.
In order to increase the effectiveness of sterilization, it is in fact necessary to raise the maximum temperature of the process, with the consequence of damaging the product from the organoleptic point of view, sometimes giving it a cooked or burnt taste or reducing its natural taste and aroma. In non-alimentary products, high temperatures can destroy essential enzymes and proteins.
Furthermore, in indirect-transmission systems the heat is transmitted from outside inward, so that it is necessary to increase the temperature of the exchange surface in order to destroy the micro-organisms even in the innermost regions. This can produce a partial non-uniformity and ineffectiveness of the process.
In direct-exchange systems, the heating medium is generally constituted by steam, which has the disadvantage of condensing inside the product itself.
Electronic preservation apparatuses having no exchange surfaces have recently been provided and are based on the following principle.
As is known, micro-organisms, like all living organisms, are poor heat and electrical conductors. Because of this, the application of heat to these organisms is difficult and slow, and also occurs unevenly. In practice, due to their low electrical conductivity, micro-organisms behave like dielectric particles which align in an external magnetic field.
FIGS. 1a and 1b schematically illustrate the structure of a pathogenic micro-organism A, for example constituted by a unicellular organism which has a generally spherical shape, when it is not subjected to any magnetic or electric field. If the micro-organism A is placed between the pates B and C of a capacitor suitable for generating an electric field with lines of force which are substantially perpendicular to the surfaces of said plates, it is observed that its approximately circular contour deforms and assumes the configuration A' due to the migration of charges with opposite signs toward its ends. Therefore, part of the energy of the field is transferred to the micro-organism as deformation work and part is transformed into kinetic energy, which increases the micro-organism's molecular agitation and therefore its temperature. Collisions due to molecular agitation and the deformation work tend to weaken or break the atomic bonds of the molecules of the microorganism A, altering its structure irreversibly. In FIG. 1b, the orientation of the electric field generated by the plates B and C is reversed, and consequently the molecular dipole also undergoes a change in shape and orientation, assuming the configuration A" which is symmetrical to the preceding one. High-frequency oscillation of the electric field generated by the plates B and C therefore produces corresponding structural modifications of the molecules of the pathogenic microorganisms, accompanied by mild heating, causing their complete degeneration at certain resonance frequencies.
By using the above described physical principle, electronic preservation methods entail the immersion of the product in a high-frequency alternating electric or electromagnetic field for a time sufficient to cause the structural degeneration of the pathogenic microorganisms.
The Japanese patent application, publication No. 2-211855 filed on Feb. 10, 1989, describes a method and an apparatus for sterilizing an alimentary liquid by irradiation with high-frequency electromagnetic waves.
In this known method, the radiation is constituted by microwaves at frequencies higher than 1 GHz emitted by a magnetron oscillator and are transmitted axially inside a waveguide, with very short irradiation times in the range of a few seconds. Due to the high frequency and to the limited wavelength of the electromagnetic waves, shielding is necessary in order to protect the personnel that works in the neighboring area. The intense heating caused by the microwaves furthermore makes it necessary to perform extremely short treatments in rapid succession, each of which is followed by intense cooling in order to keep the product below the temperature at which its organic and organoleptic properties change.
U.S. Pat. Nos. 2,576,862 and 3,272,636, FR-A-2 547 732 and DE-A-2 628 234 describe other methods and apparatuses which use electromagnetic waves with frequencies comprised within the ranges of microwaves and/or radio frequencies. These known methods and apparatuses are applied to already-packaged products and always require appropriate shielding against emissions which are harmful to the human body.
Furthermore, since the destructive action of the alternating electromagnetic field affects not only the pathogenic microorganisms but also the active principles which determine the organoleptic properties of the products to be preserved, these known methods and apparatuses change said organoleptic properties, reducing the value of the active principles.